Methyl mercaptan in particular is an industrially important intermediate., e.g. for the synthesis of methionine as well as for the synthesis of dimethyl sulfoxide and dimethylsulfone. Today, it is produced predominantly from methanol and hydrogen sulfide by reaction on a catalyst of aluminum oxide. The synthesis of the methyl mercaptan generally takes place in the gas phase at temperatures of between 300 and 500° C. and under pressures of between 1 and 25 bar.
Apart from the methyl mercaptan formed, the reaction mixture contains the unreacted starting substances and by-products, such as e.g. dimethyl sulfide and dimethyl ether, as well as the gases that are inert in the context of the reaction, such as e.g. methane, carbon monoxide, hydrogen and nitrogen. The methyl mercaptan formed is separated from this reaction mixture.
For the economic efficiency of the process, the highest possible selectivity is demanded in the catalytic reaction of methanol and hydrogen sulfide to form methyl mercaptan, in order to keep the costs as low as possible for the separation of the methyl mercaptan formed from the reaction mixture. A major cost factor here is represented in particular by the energy input for cooling the reaction gas mixture to condense the methyl mercaptan.
To increase activity and selectivity, potassium tungstate or cesium tungstate is conventionally added to aluminum oxide as the support. The tungstate is generally used in quantities of up to 25 wt. %, based on the total weight of the catalyst. An improvement in activity and selectivity is also obtained by increasing the molar ratio of hydrogen sulfide to methanol. Molar ratios of between 1 and 10 are conventionally used.
However, a high molar ratio also means a high excess of the hydrogen sulfide in the reaction mixture and thus the need to circulate large quantities of gas. To reduce the energy input required for this, the ratio of hydrogen sulfide to methanol should therefore deviate from 1 only slightly.
U.S. Pat. No. 2,820,062 relates to a process for the production of organic thiols, in which a catalyst of active aluminum oxide is used, to which potassium tungstate has been added in a quantity of 1.5 to 15 wt. %, based on the weight of the catalyst. With this catalyst, good activities and selectivities are achieved at reaction temperatures of 400° C. and molar ratios of 2. This US Patent Specification mentions various ways of introducing the potassium tungstate into the aluminum oxide. Thus, impregnation processes, co-precipitations and pure mixings are mentioned as being applicable. Little importance is attached to the actual production of the catalyst for the economic efficiency of the process for methyl mercaptan synthesis.
In EP 0 832 687 B1, the advantages of using cesium tungstate (Cs2WO4) instead of potassium tungstate (K2WO4) as promoter are described. Thus, by using cesium tungstate, increased activity can be achieved with good selectivity at the same time.
By increasing the cesium tungstate concentration to up to 40 wt. %, the selectivity towards methyl mercaptan can be increased to up to 92% without the activity being disproportionately impaired.
According to the general opinion, the best selectivity is achieved with catalysts in which the alkali/tungsten ratio equals 2:1 (A. V. Mashkina et al., React. Kinet. Catal. Lett., vol. 36, No. 1, 159-164 (1988)).